The present invention relates to a weldable connecting member for connecting or joining thermoplastic pipe elements, which has a thermoplastic body, in which a heating element is embedded for generating heat and which is used for welding the body to the pipe element or elements located in the welding area.
The joining of thermoplastic pipe elements with the aid of a thermoplastic connecting member provided with a heating element is generally known. The term pipe elements is understood to mean pipelines, fittings and shaped parts, which are joined together by the connecting members to form pipe systems and the like. The connecting members are frequently constructed as electric welded sleeves and as electric weldable spot drilling and connecting clips. However, the connecting members can also be used for connecting elements other than pipe elements, that is, for joining together rods. However, the use of weldable connecting members in building pipelines is one of the most frequent applications.
In order that the joining of the pipe elements by means of such connecting members takes place reliably, it is necessary to check the welding operation. The quality of the weld is essentially dependent on the correct temperature in the welding area and the welding pressure occurring during the welding process, that is, the pressure on the material in the welding zone which has been plasticized by the heat supplied. This results from the fact that the connecting member is given a shrinkage reserve which, during the plasticizing of the welding area by the heat supplied, leads to a shrinkage of the connecting member. This shrinkage, linked with the volume increase of the material on heating the welding area, leads to said pressure rise in the latter.
As all circumstances preventing the reaching of the correct welding temperature and pressure result in a quality reduction of the welded joint, over a long period now the joining members have been provided with additional indicating means enabling conclusions to be drawn on the weld quality.
In one known indicator a color change point is used, which is applied to the outside of the sleeve surface and in the case of adequate heating of the connecting member body undergoes a color change. In order to achieve an improved temperature indication, it is known (Swiss Pat. No. 553 368), to place the color change point on a reduced wall thickness zone. However, even in this case it is only possible to determine the temperature, but not the welding pressure and there is also uncertainty regarding the possibility of a color change resulting from external thermal influences.
As the temperature indication on the connecting member does not in itself enable reliable conclusions to be drawn regarding the quality of the weld, solutions are also known in which the pressure occurring in the welding area of the weld is made visible. For this purpose, weak points in the connecting member body can be provided, which bulge as a result of the welding pressure which occurs. It is also known (Swiss Pat. No. 632 078) to provide recesses, whose bottom terminates in the vicinity of the welding area. During welding, said recesses fill with more or less plastic material and consequently indicate whether there is an adequate welding pressure with corresponding temperature in the welding area. A known improvement of this quality indication for the weld (Swiss Pat. No. 601 719) consists of placing a pin in the bore. When the welding pressure occurs, the pin rises over the sleeve surface and is consequently readily visible. This construction has proved particularly appropriate in the case of relatively thin-walled connecting members. The projecting pin constitutes a relatively accurate measure of the temperature and welding pressure in the welding area. However, in the case of higher welding pressures, as conventionally occur with thick-walled connecting members, said construction suffers from the disadvantage that it is difficult to establish the depth of the bore. If the bottom of the bore is too close to the welding area, the pin movement takes place prematurely, but if the bottom of the bore is remote from the welding, area there is no pin movement, although the welding pressure has occurred.